The horn is a sound the player can cause a vehicle to emit for warning purposes. A horn is available for almost every land vehicle in the GTA series since Grand Theft Auto III.

Using the horn has no effect on traffic, but it does on pedestrians since the GTA III Era; pedestrians would crudely spin around to face the player and then do offensive gestures at the player, and in GTA IV Era the player would get a more convincing reaction of surprise, shock or anger. It is utilised in the mission (such as Wear Flowers in Your Hair) and used to attract other characters such as friends, characters in missions and prostitutes. In some few missions where the player is following another character stealthily, using the horn excessively will result in the player being noticed by the target character and the player will usually fail the mission.

Some horn sounds are shared between vehicles within the same game, whilst others are unique. In GTA IV, a severe crash to the front of the vehicle will often cause the driver to be killed and they will be either launched through the windscreen or impact with the wheel, and be slumped against the steering wheel and sound the horn.

Sirens

Most Emergency Vehicles have sirens in addition to the horn. Holding the horn button will sound the regular vehicle horn, whilst tapping it will enable and disable the siren. Some vehicles also have a secondary siren noise sounded by holding the horn button whilst the siren is enabled. And in the case of GTA IV police vehicles, toggling the horn button wile having the secondary horn engaged, will result in a third high frequency somewhat riot detouring siren.

Where fitted, flashing lights or lightbars also activate when a siren is sounded. It is not possible to turn on the lightbar without turning on the siren, although in most games, if the player leaves the vehicle the lightbar continues to operate without the siren. When the player damages the light-bar on the roof of service vehicle, the siren will function incorrectly making a certain broken sounding siren instead of the regular. Interestingly, using the Health, Armour and Weaponscheat, which also repairs the current vehicle, fixes the lightbar but the siren continues to function incorrectly until the player turns it off and back on.

In some games, the horn button is sensitive, and trying to turn on the siren may cause the horn to sound instead. This was fixed in later games by forcing the player to hold the button for a longer time in order to sound the horn, but this comes with the side-effect of not sounding the horn at the intended time.

Trivia

In GTA IV, sounding the horn in the tune of Shave and a Haircut will lead to another driver responding with two beeps (two bits).